This invention relates to a signal suppression system which is used, accompanied by spectrum analyzers, for measuring distortion factors of electronic components such as amplifiers, filters, and signal transmission lines, or signal reproduction devices, such as compact disc players or video tape recorders.
A spectrum analyzer is utilized in order to analyze the frequency of signals. The spectrum analyzer can measure distortion factors of higher order frequency elements by comparing them with a fundamental wave.
Also, by using a two input type spectrum analyzer, a comparison between a stimulus, which is provided to a device under test (DUT), and its response output is available. In addition, by analyzing correlations between the stimulus signal and the response signal, response characteristics of the DUT, that is, a transfer function, will be determined.
Due to the improvement of microcomputers, a digital spectrum analyzer which internalizes a FFT (Fast Fourier Transformation) or a DFT (Discrete Fourier Transformation) and various kinds of signal processing functions, has been developed. Through the use of this digital spectrum analyzer, frequency analysis, distortion factor measurements and transfer function measurements are easily, automatically and accurately accomplished.
In taking measurements of this kind, a signal generator is used as a stimulus. This signal generator is generally composed of a ROM, in which various waveform data are stored, and means for reading out data from the ROM. By reading out desired waveform data from the ROM with a desired speed, signals having expected waveforms and frequencies are available.
Therefore, as shown in FIG. 1, a signal generator 1 outputs a stimulus signal 2 which satifies frequency, waveform and other requirements, and provides the signal 2 to a DUT 3. A two-input spectrum analyzer 4 receives two signals; the signal 2, which is also being supplied to the DUT 3, and a response signal 5, which is outputted from the DUT 3. The spectrum analyzer frequency analyzes both signals to indicate a frequency spectrum, which is displayed on a display 4A. In addition, the spectrum analyzer 4 can, for example, obtain an amplitude ratio and a phase difference between a fundamental wave in the stimulus signal 2 and the response signal 5 and an amplitude ratio of the fundamental wave of the stimulus signal 2 and harmonic waves of the response signal 5. From the results of the above frequency analysis the spectrum analyzer can also obtain transfer functions and distortion factors of the DUT 3.
In order to measure, for example, distortion factors of the DUT 3, a sine wave which has an extermely small distortion ratio is applied to the DUT 3. Then, the ratios of the resulting fundamental element and harmonic elements are obtained. The DUT 3 can be an audio amplifier or a speaker or a video amplifier, and these products have extremely small distortion. When the distortion factor of the DUT 3 is small, the output level of the harmonic wave elements will also be small. However, if a fundamental wave level is large, accurately measuring the level of the harmonic wave elements becomes very difficult and costly, since very wide measuring of the dynamic range is required.
In order to solve the above-described problems, a high pass filter can be employed at the output of the DUT 3 to remove the fundamental wave. However, if a filter is inserted in series with the DUT 3, the true distortion factor will not be calculated. This is because the amplitude and phase characteristics of the filter are added to the amplitude and phase characteristics of the resulting signal from the DUT 3.